Before coating a metal surface with a paint, the surface is generally treated with a zinc phosphate solution in order to enhance corrosion resistance and the adhesive properties between the metal surface and a paint layer thereon. However, corrosive environments are getting worse in automobiles, because a large amount of salt is spread over roads in winter. A primer coating is accordingly changed from anionic electrocoating to cationic electrocoating, and the surface treatment of the zinc phosphate solution is improved to suit to the cationic electrocoating method.
In the cationic electrocoating method, however, residual stress remains in a cured film so as to deteriorate adhesive properties, thus deteriorating corrosion resistance. In other words, the volume contraction at baking a coating forms internal stress in coating and deteriorates the adhesive properties between the metal surface and the cured film. The deterioration of the adhesive properties does not appear under conventional salt spray test conditions. But, if a salt spray test is conducted at more severe conditions, corrosion resistance significantly declines.